[css-color-5] Make colorspace argument mandatory for color-mix()

Closes w3c#6051
Also commented out an example that used adjusters

Author: LeaVerou

css-color-5/Overview.bs

@@ -121,14 +121,15 @@ Mixing colors: the ''color-mix()'' function {#color-mix}
 
 
 <pre class='prod'>
-	<dfn>color-mix()</dfn> = color-mix([ in <<colorspace>> ]? <<color>>{2} && [ <<percentage>> ]?)
+	<dfn>color-mix()</dfn> = color-mix( in <<colorspace>> <<color>>{2} && [ <<percentage>> ]?)
 </pre>
 
 If a percentage is omitted, it defaults to 50% (an equal mix of the two colors).
 
 The result is produced via the following algorithm:
 
 1. If a percentage is omitted, it defaults to 50% (an equal mix of the two colors).
+If the color space is omitted, it defaults to 'lch'.
 2. Both colors are converted to the specified [=colorspace=].
 If the specified colorspace has a smaller gamut
 than the one in which the color to be adjusted is specified,
@@ -144,7 +145,7 @@ and 50% produces an equal mix of both.
 	This example produces a mixture of 40% <span class="swatch" style="--color: peru"></span> peru
 	and 60% <span class="swatch" style="--color: lightgoldenrod"></span> lightgoldenrod.
 
-	<pre class="lang-css">color-mix(peru  40%, lightgoldenrod)</pre>
+	<pre class="lang-css">color-mix(in lch peru 40% lightgoldenrod)</pre>
 
 	The mixing is done in ''lch()'' colorspace.
 	Here is a top-down view, looking along the neutral L axis:
@@ -180,7 +181,7 @@ and 50% produces an equal mix of both.
 	keeps the intermediate colors
 	as saturated as the endpoint colors.
 
-	<pre class="lang-css">color-mix(in lch, teal 65%, olive);</pre>
+	<pre class="lang-css">color-mix(in lch teal 65% olive);</pre>
 
 	<figure>
 		<img src="images/CH-mixing3.svg" style="width: 50vmin">
@@ -388,7 +389,7 @@ Array(3) [ 0.524461282381659, 0.4582102541032295, 0.6295269395052022 ]
 which is rgb(52.446% 45.821% 62.953%)
 
 
--->
+
 
 <div class="example">
 	This example produces the mixture of
@@ -424,7 +425,7 @@ which is rgb(52.446% 45.821% 62.953%)
 	  * result is <span class="swatch" style="--color: rgb(52.446% 45.821% 62.953%)"></span> lab(52% 13.572 -21.154) which is rgb(52.446% 45.821% 62.953%)
 
 </div>
-
+-->
 
 <!-- IMPLEMENTATIONS